This invention relates to scintillation detector arrays which are useful in tomography or other related industrial applications. More particularly, this invention relates to a structure for enhancing the channeling of the optical output of a scintillator body, excited by x-ray radiation, to photoelectrically responsive devices.
A scintillator is a material which emits electromagnetic radiation in the visible or near-visible spectrum when excited by high energy electromagnetic photons such as those in the x-ray or gamma-ray regions of the spectrum, which region is hereinafter referred to as the supra-optical frequency region. Also as used herein the term "light" refers not only to the visible region of the spectrum but also to those near-visible regions which encompass the radiation emitted by certain scintillators in the infrared or ultraviolet regions. In typical tomographic or industrial applications, the light output from scintillator materials is made to impinge upon photoelectrically responsive materials in order to produce an electrical output signal which is in direct relation to the intensity of the initial x-ray or gamma-ray bombardment which has been modulated by causing the supra-optical photons to pass through the body being studied.
In general, it is desirable that the amount of light output from the scintillators be as high as possible for a given amount of x-ray or gamma-ray bombardment. This is particularly true in the medical tomography area where it is desired that the energy intensity of the x-ray be as small as possible to minimize the danger to the patient. For this reason, as large an amount as possible of the optical output of the scintillator should be directed to the photoelectrically responsive transducer.
In the past, single crystals of scintillator material, such as cesium iodide (CsI), have been proposed and used in scintillation detector arrays. Single crystals are not always available, or are too costly. Hence, scintillator bodies prepared from phosphor powders have been proposed. However, the optical opacity of these scintillator bodies has prevented an optimal amount of light from reaching the photoelectrically responsive detectors. The amount of detectable optical output has been limited to that which is generated by x-ray excitation in or near the surface regions of the scintillator body and also that optical output generated deep within the scintillator body which can escape the body albeit in an attenuated level.
However, recent inventions have produced scintillator bodies from which light escape is greatly enhanced. For example, in application Ser. No. 853,085, filed Nov. 21, 1977 by Cusano et al., which is assigned to the same assignee as this application and which is thereby incorporated herein by reference, discloses transparent scintillator bodies produced by hot-pressing and hot-forging. The transparency of these bodies permits a greater amount of optical output than occurs in opaque scintillation detectors. Also, in application Ser. No. 853,086, filed Nov. 21, 1977 filed Cusano et al. and which is assigned to the same assignee as this application and which is hereby incorporated herein by reference, there is disclosed two embodiments of a distributed phosphor scintillator structure in which the phosphor is distributed in either a continuous or a layered fashion within or between a transparent matrix material. Here too, the optical output more readily escapes the scintillator body for detection, than in the case of an opaque scintillation material. Similarly, in application Ser. No. 863,876, filed Dec. 23, 1977 filed by Cusano et al, which is assigned to the same assignee as this application and which is also hereby incorporated herein by reference, a distributed phosphor scintillator body is disclosed in which the phosphor is distributed continuously throughout a transparent matrix material which is matched to the phosphor material in its index of refraction. As discussed therein, this also further enhances the escape of optical energy from the scintillator body whereby it is more readily detected.
Another negative feature of prior scintillation detector arrays is that the photoelectrically reponsive detectors are typically mounted at the rear of the array in the direct path of the x-ray or gamma-ray beam. This is undesirable in that prolonged exposure to such bombardment of photoelectrically responsive detection devices such as silicon photodiodes results in a deterioration in their performance and efficiency. Furthermore, the detectors themselves will show response due to direct x-rays which pass through or by the edges of scintillators and cause poor channel-to-channel signal uniformity.